The invention relates to a method for filtering a liquid, in which at least one filtering aid is added to the dirty liquid to be filtered and a higher degree of purity is achieved.
A method for cleaning liquid cooling lubricants is known from the German patent 35 37 384. For this method, a pre-coated filter is disposed in a cycling system and filtering aids with a highly surface-active and adsorbing substance are used in the pre-coated filter. These filtering aids are, for example, conventional commercial fuller""s earths. It is a disadvantage of the system that, to begin with, a filter cake of the filtering aid must be built up, so that a sufficient degree of filtration results. It must therefore be expected that the input of filtering aid will be high. A further disadvantage can be seen therein that a precise dosing of the pre-coating layer is a problem because of turbulences and that the filtration result is therefore subject to high fluctuations under some circumstances.
It is an object of the invention, to provide a method and an apparatus for filtering a liquid, for which the amount of filtering aid used is to be minimized and a high filtration quality is to be achieved in filtration plants.
This objective is accomplished by the by the method and apparatus as described and claimed hereinafter.
The advantage of the method lies therein that an initial pre-coating is not required. Instead, filtering aid is added to the dirty liquid that is to be filtered. By these means, a high degree of purity can be achieved.
According to a refinement of the invention, it is proposed that the open-pore character be assured by appropriately dosing the filtering aid. For this measure, especially an appropriate particle size of the filtering aid or an appropriate compression strength of the filtering aid is especially important for avoiding any caking.
According to a refinement of the invention, the filtering aid can be dosed in continuously or discontinuously. The inoculation advantageously is discontinuous if the input of dirt is very low or if the service life of a filtration plant is to be prolonged. Filtering aids can also be added when the plant is not in operation and it is only necessary to maintain the quality of the liquid.
Pursuant to a further development of the invention, the filtering aid is pre-mixed with the dirt components or with the dirty liquid, so that a better distribution of the filtering aid takes place. Advantageously, either crushed corn or also cellulose can be used. Especially the inoculation with cellulose achieves a filtration quality, which is better than that achieved with the previously used pre-coating with diatomaceous earth or fuller""s earth. Since it is injurious to health, special diatomaceous earth should be avoided in the future.
The fact that the residue can be disposed of easily is a further advantage of the use of crushed corn or cellulose. Cellulose, for example, burns almost without leaving any residue.
It has turned out that the invention can be employed advantageously in different filtration plants. The filtering aid increases the lifetime of the liquid, for example, of an emulsion. In addition, a degree of filtration of less than 15 xcexc is achieved. The filtration plants include, for example, gravity belt filtration plants, pressure belt filtration plants or vacuum filtration plants.
In a further variation of the method, a filtration plant is pre-coated only partially, that is, the filtering aid covers only a certain region of the effective filter surface.
As a result, a sort of partial flow filtration results. The essential advantage is seen therein that two filters can be integrated into a single filtration system. Likewise, it is possible to carry out the pre-coating at different densities. By these means also, filters with different degrees of filtration are formed.
An additional variant of the invention is represented by the basic pre-coating of the filtering aid. In this variant, initially only the filtering aid is introduced. Subsequently, the dirty liquid is added. This basic pre-coating has the advantage that a high degree of filtration is achieved at the very start of the filtration process. Quite generally, the invention exhibits the following improvements in comparison to previously known methods:
reduction in the consumption of filtering aid;
avoidance of the use of filtering aids, injurious to health, such as diatomaceous earth in pre-coated filtration plants;
increase in the quality of the filtration;
replacement of pre-coated filtration plants as state of the art technology for high-degree and very high-degree filtration requirements when cooling lubricants are used, both in the case of oil as well as in the case of suspensions;
the possibility of working up spent oils;
prolonging the maintenance interval for cooling lubricants;
reducing the costs of cleaning storage containers for cooling lubricants;
reducing the problem of disposing of waste materials;
reducing the costs of the disposal.
These and further distinguishing characteristics of preferred further developments of the invention are evident from the claims as well as from the specification and the drawings. The individual distinguishing features can be realized by themselves alone or in the form of sub-combinations of several for the embodiment of the invention and in other fields and represent advantageous, patentable developments, for which protection is claimed here.